This invention relates to plasma-generating devices. More specifically, this invention relates to shower head magnetron-type plasma-generating devices containing dual shower heads and capable of sustaining plasmas of more uniform density than plasmas generated by previously known magnetron-type plasma-generating devices and taking less space in a plasma-generating device.
Magnetrons have been known in the art for a long time and have been used, for example, in etching, surface modification, and plasma-enhanced chemical vapor deposition (xe2x80x9cPECVDxe2x80x9d). PECVD devices are also known in the art. Examples of PECVD devices can be found in U.S. Pat. Nos. 5,298,587; 5,320,875; 5,433,786; and 5,494,712, (collectively xe2x80x9cHu et al.xe2x80x9d). Shower head magnetrons are also known. In these, the reaction gases are transferred through and out of one face of the magnetron in a manner similar to a shower head used to disperse water as a spray. One embodiment of a shower head magnetron is taught in the above cited U.S. patents.
Magnetrons are a class of cold cathode discharge devices generally used in a diode mode. In a plasma-generating device, a plasma is initiated between a cathode and an anode at pressures in the m Torr range by the application of a high voltage, which can be either dc or rf. The plasma is sustained by the ionization caused by secondary electrons emitted from the cathode due to ion bombardment which are accelerated into the plasma across the cathode sheath. What differentiates a magnetron cathode from a conventional diode cathode Is the presence of a magnetic field. The magnetic field in the magnetron is oriented such that a component of the magnetic field is parallel to the cathode surface. The local polarity of the magnetic field is oriented such that the ExB drift paths of the emitted secondary electrons form a closed loop. Due to the increased confinement of the secondary electrons in this ExB drift loop, compared to a dc or rf diode device, the plasma density is much higher, often by an order of magnitude or more, than a conventional rf or dc diode plasma. The result of the high plasma density and its proximity to the cathode is a high current, relatively low voltage discharge.
Hu et al. teach a method of forming a flexible corrosion and abrasion resistant coating onto a substrate surface. In one embodiment of the method taught in Hu et al., a PECVD method, preferably using a shower head magnetron cathode, is used to initiate the polymerization reaction of an organosilicone compound and oxygen employing an adequate power density, in the presence of a substrate having a suitable surface to cause the polymerizabon product of the plasma process to adhere to the substrate surface. In Hu et al., a single face shower head magnetron utilizing magnets having sufficient strength to provide at least 100 gauss is used.
It is also known in the art that when using a magnetron in a process to coat a substrate such as in a PECVD process or sputtering process, it is difficult to obtain a coating of uniform thickness and quality. One aspect of quality is uniform chemical composition of the coating both in thickness and width directions. In some prior art techniques, to get a coating of uniform thickness and quality the substrate must be moved relative to the electrodes. This is especially true for large substrates. Moving the substrates relative to the electrodes can cause a decrease in throughput.
The present invention allows for mom uniform (thickness and quality) coatings to be obtained more easily than do devices of the prior art, especially on large substrates.
In one aspect, the present invention is a shower head electrode for a magnetron plasma-generating apparatus containing multiple magnets wherein dual opposite parallel faces of the electrode comprise a shower head design, such that reactant gases can be discharged into a reaction chamber from at least two surfaces. The magnets also provide magnetic fields to both faces of the electrode. Preferably the multiple magnets are positioned internally in the shower head such that like magnetic poles of said magnets all face in the same direction and serve to provide magnetic fields on each face of the electrode. In the preferred mode, the magnets are placed in a position and distance from each other such that each magnet produces a magnetic field between the opposite magnetic poles on the same magnet. Each magnetic field has a component parallel to the surface of the electrode. electrodes of the preferred mode of the invention have a higher number of closed loop ExB drift paths per number of magnets than electrodes where each magnetron is composed of three magnets, where the center magnet has the pole opposite to each outside magnet. By aligning all magnets so that the same poles face in the same direction, each magnet becomes a separate magnetron. Electrodes of the preferred mode are capable of producing a more uniform plasma across the surface of an electrode. In addition, in the preferred mode the electrodes produce plasmas of greater effective volume than electrodes where the magnets are arranged with opposite poles facing each other. According to the present invention, large numbers of magnets (that is, two or more) can be aligned in various configurations so as to create large electrodes capable of producing large, more uniform, plasmas.
In another aspect, the present invention is an improved plasma-generating device utilizing at least one dual face electrode of the present invention. In still another aspect, is a plasma-generating device containing at least one shower dual face electrode of the present invention facing, with a space between and opposite each face, a shower head electrode having only one shower head face. In another mode, two shower head electrodes having only one shower head face are placed in a plasma-generating device having placed between the two at least two dual face shower head electrodes of the invention. In another embodiment, more than one dual face electrode is disposed between two single face electrodes.
In yet another aspect, the present invention is an improved method of forming a plasma and an improved method for coating various substrates.
In one embodiment of the present invention, the electrode is a planar electrode comprising two shower head faces essentially parallel to each other and facing in opposition directions and at least one magnet, preferably more than one magnet, positioned such that like poles of said magnets are in a single geometric plane parallel to the geometric plane of the planar electrode and the polarity of said magnets is perpendicular to the geometric plane of the planar electrode, each magnet producing a magnetic field having a component parallel to the geometric plane of the electrode.